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cathay747

PMDG 747 VNAV problems

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So I was flying the queen one day with the FMC and after a bit of time after takeoff, I activated LNAV and VNAV but then I discovered that I was not climbing over FL100 (way off my cruising altitude of FL290) but my speed did increase as programmed. I then discovered that my altitude knob was set at 10000 feet. Upon discovering that, I changed the altitude knob to 29000 but the altitude did not change. I was baffled so I fiddled with the buttons and pressed FLCH. The plane then pitched up violently (more than 20 degrees pitch) and it climbed to the cruising altitude in my FMC. I would be most grateful if anyone explain to me this weird behaviour and the solution to this.


Kingston Chiu

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Upon discovering that, I changed the altitude knob to 29000 but the altitude did not change.

 

This is correct.  You must hit ALT INTV for it to continue.  In order to do this, you must "press in" by clicking the middle of the altitude wheel.

 

Note the AP mode in your FMA (the top of your PFD, or "primary flight display").  Since you didn't roll the altitude higher prior to it hitting the limit in the window, the AP went from VNAV SPD mode, and into VNAV ALT.  VNAV ALT is an ALT HOLD mode, where it will remain at that altitude until "cleared higher."  It won't move away from the altitude immediately by rolling the altitude wheel to a higher altitude to allow the crews to make changes to the correct altitude before telling it to climb.  After changing the altitude, press the altitude wheel in to continue the climb in VNAV SPD mode.

(Replace PATH for SPD, as necessary, depending on the departure profile/constraints)

 

 

 


I was baffled so I fiddled with the buttons and pressed FLCH. The plane then pitched up violently (more than 20 degrees pitch) and it climbed to the cruising altitude in my FMC.

 

Pitched violently and pitched up to 20+ degrees of pitch are different concepts.

 

Pitched violently means it did so quickly and abruptly.

Pitched up to 20+ degrees is not violent, depending on how you allowed it to operate as the pilot.  FL CH is a pitch for speed mode.  This means that the AT will command full rated thrust (depending on the current thrust limit), and then adjust pitch to maintain the commanded speed.  If 20+ degrees of pitch were needed to either maintain or decrease the speed to the commanded speed, then that's just fine.  It's a bit extreme, but depending on the circumstances, I wouldn't call it weird, incorrect, or a problem.

 

To me, it just sounds like the plane went over the FL CH speed and was trying to recapture it.

 

 

 


I would be most grateful if anyone explain to me this weird behaviour and the solution to this.

 

The solution to this is to learn more about the plane and how to use its autopilot modes.  There are four type rating lessons on the PMDG Site > Downloads page.  Have a read through them.


Kyle Rodgers

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Thanks for your detailed response.

Moreover, I would like to ask that recently I have discovered in pure autopilot mode (no LNAV VNAV that stuff) the healding selector is a bit off course. For example, when I select 073 in the heading selector and press select, the plane always goes a bit off and remains at 075 for planes with PW engines and 074 for GE engines.

I would be grateful if someone can explain and provide a solution as I have already installed the whole plane once and the problem is still there.


Kingston Chiu

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Thanks for your detailed response.

Moreover, I would like to ask that recently I have discovered in pure autopilot mode (no LNAV VNAV that stuff) the healding selector is a bit off course. For example, when I select 073 in the heading selector and press select, the plane always goes a bit off and remains at 075 for planes with PW engines and 074 for GE engines.

I would be grateful if someone can explain and provide a solution as I have already installed the whole plane once and the problem is still there.

 

The ND shows track not heading. The nose of the plane may be pointing at 073, but any wind can cause the track to be different. You can show HDG on the ND by changing to VOR or APP dislpay instead of MAP.

 

The compass segment at the bottom of the PFD always shows current heading plus the little white bar shows drift angle/ground track.

 

Hope this helps.


Peter Schluter

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Hey guys I don't know if it's me but lately I've been flying the pmdg 747, and everything is going according to plan on the fmc, but then once I'm at 10000 feet, my speed all of a sudden goes to 340 knots,. I have VNAV and LNAV and A/T on and armed and autopilot is armed as well. Anyone know the problem? 


Thanks.


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Have you set the FMC correctly, mainly the PERF INIT page, and ensured that no random keypress/joystick button isn't somehow disengaging VNAV?


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Have you set the FMC correctly, mainly the PERF INIT page, and ensured that no random keypress/joystick button isn't somehow disengaging VNAV?

 

I answered him in the other thread. I'm pretty sure he's simply referring to the speed limit getting cancelled now that he's above 10,000.


Kyle Rodgers

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